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I also created two lists on Twitter following wireframing and prototyping tools, which you might consider following to see if those companies integrate support for those platforms. Since Windows Phone 7 hasn't launched yet, though, a lot of them haven't moved into that space yet.

I've preferred Fireworks for a long time now, but recently I moved to OmniGraffle (since Fireworks started to get really buggy and slow on my Mac). I've quite happy with the choice so far. Here's some very nice stencils for iPhone and iPad which includes both real graphics and grayscale versions for OmniGraffle.

Typically you start out with grayscale wireframes and add some graphics if really necessary. Personally I work faster with grayscale and it makes the wireframes stick to describing features and navigation, not colors.

I love Cacoo; it's my go-to for simple wire-framing and they have great options for iPhone, Android, etc. I've used other tools, but this has been the best for me, to mock something up before building it myself or with my friends.

You see, there are many tools. The right choice always depends on your previous experience and skills and obviously the context of your task. What do you want to show and and to who.

Additionally to the list I can recommend:
- if you want to explore all kinds of transitions, check Adobe Catalyst (or Flash)
- if you need to create specifications afterwards, the benchmark currently is Axure RP6

Actually, you can use whatever you have, as long as you know what you want to do and who is the audience of your deliverable ;)

Axure all the way! I have spent countless hours getting a prototype to work on mobile. The problem with every other program I have used is that they dont handle detailed forms. I had to prototype a workout form on the phone. Axure could handle the form 'remembering' the users choices when bouncing around from screen to screen. (ie: When the user selected 'activity' and they go to the next screen with the list of activitys, they could choose 'Run' and have that bounce them back on the previous screen with 'Run' prefilled. They could then choose the sub-activity, lets say 'treadmill run'. It would then pop me back on the original form screen with 'Run' and 'Treadmill Run' populated on the form.) Some other programs would handle this as a dropdown, but of course this is handled differently on the phone. Axure is the only program that would support this.

There is also a setting in the newer version of the software that allows you to adjust the viewpane of the prototype so that it looks like an actual app on the phone. It supports functions like 'tap' that are mobile functions.

On the site, there is a free trial. There are many widget libraries that are supported, even mobile UI elements.

1) Notepad for rapid paper prototyping of iPhone Apps GUI elements are on separated post-its. After prototyping you can take a picture of your design and send it to the UXPin App. It will be auto-converted for digital, fully editable, wireframe.

2) UXPin App - wireframing tool with full live collaboration (people can edit the same prototype in the same time) and ability to recognize paper prototypes from UXPin Notepads. You can wireframe iOS and Android apps. We're thinking about Phone 7.

I don't want it to be just a marketing post so - please remember that Apple itself got nice SDK with great tool XCode 4. You can quickly create UI there using traditional drag&drop method.